Like-cells associate to form various tissues, and it has been demonstrated that this specificity resides in the plasma membrane. Membrane bound tissue specific antigens and enzyme substrate mechanisms have been considered to be involved in this phenomena, but these explanations are incomplete. It is proposed that like-cells identify each other by specific complementary cell-surface patterns. This complementarity results from cell division. That is, when cells divide, they give rise to a mirror image of themselves. These complementary patterns are the result of the association of histocompatibility antigens (HCA) with non-histocompatability components in the plasma membrane. This proposal is based on topographic conjecture in that four colours are sufficient for colouring all maps drawn on a plane or sphere so that regions that share a common boundary are of a different colour. It is further proposed that these specific cell-surface patterns can be classified as four overlapping regions: (i) homotypic - which is generally species specific, (ii) allotypic - which is variable within a species, (iii) idiotypic - which represents variation within an individual or family group, and (iv) embryotypic - which identifies the embryonic origin of the tissue.